The present invention relates to the drying of particulate bulk materials in general, and more particularly to the drying of pulverulent materials, (powders).
It is already known to dry particulate materials, especially granulated bulk materials such as, for instance, granules of synthetic plastic material, by passing heated dry air through a body consisting of the granules. The drying air heats the granules and accepts moisture which is released from the granules to carry the moisture out of the drying receptacle. In this connection, it is already known to utilize the ambient air, which is preheated to the desired temperature, as the drying medium, and then to discharge the moisturized air into the ambient atmosphere. However, it has been established that a much more economical solution is obtained when a closed circuit is provided for the drying air, that is, when the moisturized air is passed through a dehumidifying or demoisturizing arrangement subsequent to its passage through the body of the granular particulate material, and then reintroduced into the receptacle which contains the body of the granular particulate material.
As advantageous as this method and the associated apparatus may be for demoisturizing or drying granular particular material, it has been established that this method, for all intents and purposes, cannot be used in connection with pulverulent bulk materials, inasmuch as the resistance of the body of the pulverulent particulate material to the passage of the drying air therethrough is so great that only uneconomically small amounts of the drying air can be passed through the body of the pulverulent material. As a result of this, it would be possible to, on the one hand, heat the pulverulent material to the desired temperature and, on the other hand, contact the pulverulent material sufficiently well with the dry air, only in an uneconomically long period of time.
In order to avoid this disadvantageous situation, it would be possible to very considerably increase the pressure of the warm drying air so as to, in effect, transform the body of the pulverulent material into a fluidized bed through which the drying air can flow in substantial amounts. However, even this approach is disadvantageous inasmuch as certain amounts of the pulverulent material are entrained in the drying air and are carried thereby out of the confining receptacle. So, while it is true that a more rapid heating of the material and also a more rapid withdrawal of the moisture from the pulverulent material is obtained when this expedient is resorted to, it is also disadvantageous in that it is necessary to remove the entrained particulate material from the humid drying air downstream of the particulate body as considered in the direction of flow of the drying air.